1. Field of the Invention
The present invention relates to a non-aqueous electrolyte secondary battery of a cylindrical configuration, and more specifically to a negative electrode structure of the same.
2. Background Art
With the advancement in recent years of electronic apparatuses for their portability and cordless operation, secondary batteries such as nickel hydrogen batteries and lithium ion batteries gain attention as driving power sources because they are smaller in size, lighter in weight and higher in energy density.
A typical lithium ion secondary battery comprises a positive electrode made of a complex oxide containing lithium, a negative electrode containing a lithium metal, a lithium alloy or a negative electrode active material capable of inserting and extracting lithium ions, and an electrolyte.
There are also some researches reported in recent years on certain elements having the property of inserting lithium ions and theoretical capacity densities exceeding 833 mAh/cm3 in place of carbon materials such as graphite that have been used conventionally as the negative electrode material. Silicon (Si), tin (Sn) and germanium (Ge) having the property of alloying well with lithium, oxides and alloys of these elements are some examples of the elements used for the negative electrode active material having the theoretical capacity density exceeding 833 mAh/cm3. Of these materials, silicon particles and silicon-containing particles such as silicon oxide particles are studied broadly as less expensive alternatives.
However, any of these materials has such a property that its volume increases when inserting lithium ions. In the case of using a negative electrode having a negative electrode active material composed of Si, for instance, the negative electrode active material changes to a substance symbolized by Li4.4Si under a state of inserting lithium ions to its maximum level, and a ratio of increase in volume of the active material reaches up to 4.12 times.
For the foregoing reason, the negative electrode active material expands and contracts as it inserts and extracts lithium ions. There was thus a possibility that the negative electrode active material separates from the negative electrode current collector due to weakening of adhesion between them over repeated cycles of charging and discharging.
Japanese Patent Unexamined Publication, No. 2005-196970 (“patent document 1”) is directed to address the above problem, and it discloses a method of forming a negative electrode, wherein a current collector is provided with convex portions and concave portions on its surface, and a film-like negative electrode material is formed thereon in a tilting orientation with respect to a plane perpendicular to the main surface of the negative electrode material.
According to the patent document 1, the negative electrode material of a pillar shape formed in a tilting angle on the convex portions of the current collector is overlaid with a separator and a positive electrode to make an electrode group, which is then wound to complete the secondary battery of a cylindrical configuration. It indicates that this structure can distribute stresses produced by expansion and contraction of the negative electrode material due to electric charges and discharges into directions of both parallel and perpendicular to the main surface of the negative electrode material, thereby preventing wrinkles and separation from being developed.
However, it is necessary to reduce the size of the electrode group in consideration of anticipated changes in shape of the electrode group so as to fit in the battery case since its diameter increase due to the expansion of the negative electrode active material by insertion of lithium ions. This gives rise to a problem that a space inside the battery case cannot be used effectively, thereby limiting improvement of the battery capacity. Besides, a large stress is developed upon the pillar-shaped negative electrode material due to the expansion of the negative electrode group because the size of the secondary battery is restricted by the dimensions of the battery case. In addition, since the pillar-shaped negative electrode material is connected to the current collector at the convex portions, in particular, the stress is likely to concentrate on the junctions, thereby giving rise to another problem that the pillar-shaped negative material separates from the current collector. There is also another possibility that the current collector wrinkles or deforms even if the negative electrode material remains not separated. There are cases as a consequence that the cycle characteristics and reliability became impaired.